A. Technical Field
The present invention relates to a production process for an aqueous coating composition which comprises a resin emulsion. More particularly, the present invention relates to a production process for an aqueous coating composition which gives excellent storage stability, and besides, excellent performance such as improved coatability, adhesion, and water resistance.
B. Background Art
Until now, an organic solvent type coating composition and an aqueous coating composition are used as a coating composition for automotive interior parts. However, organic solvents are being strictly regulated in view of environmental conservation in recent years. The usage of the aqueous coating composition increases due to this situation.
The aqueous coating composition generally comprises a resin emulsion.
In this resin emulsion, the resin component is particulate, and the resin particle itself cannot form coating films. Therefore, a hydrophilic organic solvent having a high boiling point is generally formulated as a promoter for forming coating films.
However, the above organic solvent makes the viscosity of the coating composition increase, and makes it difficult to spray, and besides, makes the qualities of the coating film (for example, adhesion and water resistance) deteriorate.
In order to enable to use a coating composition with a high viscosity, there is a method which involves diluting this coating composition to decrease its viscosity and spraying. However, the method requires a diluting process, and besides, accompanies another problem such that the sagging performance is deteriorated.
A. Object of the Invention
An object of the present invention is to provide: a production process for an aqueous coating composition including a resin emulsion, which has excellent storage stability, and besides, excellent sagging performance, adhesion and water resistance.
B. Disclosure of the Invention
The present inventors repeatedly studied to solve the above-mentioned problems. As a result, they found that the storage stability is decreased because (1) an organic solvent combined with a coating composition as a promoter for forming coating films is permeated into a resin particle of emulsion and swells the resin particle, and therefore its particle diameter becomes greater, the volume of the resin particle in the coating composition becomes larger, and the viscosity of the coating composition increases in appearance, and (2) the resin particles make partially physical bond each other like a crosslinkage, and consequently the viscosity increases. Then, as a result, they found that the above reasons cause properties of coating films such as sagging performance and adhesion to decrease, depending on circumstances.
The present inventors believed that it is necessary for the resin particle to prevent contact with the organic solvent, and not to swell in the above way, so that the above problems (1) and (2) would not be caused, and further, they variously considered this matter. As a result, they found that an aqueous acrylic resin solution might be added to the resin emulsion in a specific ratio before adding the organic solvent as the film forming promoter. The emulsion resin is covered with the acrylic resin if the aqueous acrylic resin solution is added to the resin emulsion beforehand. Therefore, they supposed that the organic solvent would swell little emulsion resin particle even if the organic solvent is added afterward. The present invention was completed in accordance with these findings.
That is to say, a production process for an aqueous coating composition, according to the present invention, comprises the steps of:
beforehand mixing 20 to 50 parts by weight of an aqueous acrylic resin solution with 100 parts by weight of a resin emulsion, wherein the aqueous acrylic resin solution and the resin emulsion are in a state of not including an organic solvent; and
adding an organic solvent to a mixed liquid obtained by mixing the resin emulsion and the aqueous acrylic resin solution.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.